The note from Belgium's anti-terror unit to police and published by Dernière Heure newspaper, warned: "Combatants are thought to have left Syria around a week and a half ago to reach Europe via Turkey and Greece by boat without passports." It gave no specific dates.

A police officer lays flowers on June 15, 2016, near the house in Magnanville where a man claiming allegiance to the Islamic State group killed a French policeman and his partner on the night of June 13Credit:
Matthieu Alexandre/AFP Photo

"These people are thought to have split into two groups - one for Belgium, the other for France in order to commit terror attacks in groups of two," it was cited as warning.

"According to information collected, these people are thought to already be in possession of the necessary weaponry and their attack is imminent."

While Dernière Heure makes no mention of specific threats to football stadiums or other targets in France, it cites more precise information on three Belgian targets. These include " a big Brussels shopping centre, a restaurant of a American fast food chain (not localised) and a police target, such as a police station (not localised)".

Paul Van Tigchelt, one of the heads of Belgium's anti-terror body, Ocam, sought to play down the note, saying it had received similar intelligence of an imminent attack in April which came to nothing.

Mr Van Tigchelt said: "Ocam receives all sorts of information. This is raw intelligence. Our mission, in relation with our partners, is to put it into context, to analyse it and verify how reliable it is."

For the past few weeks, Ocam has kept the country on alert level three, meaning there is a "serious" risk of attack. The maximum level is four, meaning the risk is "serious and imminent".

Mr Van Tigchelt said there was no intention of raising the alert level in Belgium.

France, however, "remains the preferred target for Isil," wrote DH, citing Belgian intelligence.

A source at France's interior ministry confirmed that they had received a note from Belgian authorities and were currently reviewing the information in the alert. "We know the threat is very high," the source said. "We're reviewing all the elements (in the alert)."

While he would not confirm the specific Belgian threat, Stéphane Le Foll, the French government spokesman, said: "We know there are fighters who are coming back (to Europe)."

DH also revealed that Mohamed Abrini, the terror suspect arrested in the wake of the attacks on Brussels airport and metro in March that killed 32, has managed to communicate with the outside world despite being confined to a prison in Beveren.

A message scrawled in Arabic on an A4 piece of paper that was intercepted by a fellow inmate and handed to prison authorities reads: "Something is moving in France".

In a speech at the Elysee Palace on Wednesday President Francois Hollande warned that fighting terrorism is "a long war to wage not just in a few countries but in the whole world, everyone can be concerned".

Manuel Valls, the prime minister, said: "I said we were at war, that this war will take a generation, that it will be long",

"Other innocent people will die. It is very hard to say. People can accuse me - and I completely understand - of making the society even more fearful than it already is today with these events. But unfortunately, this is the reality. It will take a generation."

French Muslim leaders expressed horror at the police murders. Dalil Boubakeur, honorary president of France's national council of the Muslim faith, CFCM, and rector of the Paris mosque, said the time had come to lock up violent Islamists and their "deathly and barbaric ideology".

"These types of individual are moving freely around France. It can't go on any longer."